The invention relates to a method of and an apparatus for measuring the water content of flowable materials, e.g. casting sand, concrete admixtures or chemical mixtures, by determining a physical quantity or property of the moist material in contact with a measuring probe and by converting this physical quantity into a measurable electrical signal, the measuring probe being submerged in the stationary or flowing material.
Various methods and apparatuses for measuring the moisture content are known, and during recent years three methods, suitable for automatic operation, have crystallized out, namely, the conductivity method, the nuclear physical method and the di-electrical method. The method and apparatus for measuring moisture content in accordance with the invention are suitable for use in various measuring systems including the capacitive system. In the following, the method will be described in connection with the determination of the dielectric constants and the high-frequency attenuation of the measured material in the short-wave range to enable an illustrative specific method to be selected from several possible methods, without limiting the invention to the selected one.
In the production of concrete the measurement of the moisture content for example is of decisive importance, not least as regards the controlled addition of moisture and therefore as regards determination of the water-cement ratio. In many cases it is not sufficient, for the purpose of maintaining particular material properties, simply to accept a mean value based on the range over which the moisture content varies, and it is particularly advisable in automated operations to measure the actual moisture content of the materials on a continuous basis and to correct the amounts of added components accordingly.
In measuring the electrical conductivity of the material to be processed two or more electrodes are used in known methods, and the moisture content of the mixture disposed between the electrodes is determined by measuring the change in resistance. Although such conductivity measuring instruments are not expensive, the value indicated by them is disadvantageously dependent upon impurities in the material and upon temperature.
It is also known to measure the dielectric constants in conjunction with the high-frequency attenuation of a moist medium, in contact with the measuring probe, in the short-wave range.
Most of the known methods are limited to carrying out measurements in containers, silos or conveyor means, e.g. on conveyor belts. However, the dielectric is greatly altered by the degree of compacting of the material and by differences in, or absence of, homogeneity in the measuring medium. Furthermore, moisture-content values, measured in preliminary containers or trucks, are intended to be used for correcting the moisture content by adding a suitable quantity of water to obtain the required value, this operation being carried out in a following machine for preparing the material. However, when measurements are carried out in stationary containers, there arises the problem that only part of the amount of the material that is used for preparing each charge is subjected to the measurement. Since, however, many bulk materials, such as for example casting sand, exhibit considerable variations in moisture content, the determination of a value relating to only part of the material just does not provide reliable information regarding the average moisture-content of the entire quantity. Even if the measuring probe is submerged in material passing along on conveyor belts, considerable variations in the measured values are observed which are caused either by the material having caked up in a silo discharge duct or by uneven depth of layer of material on the conveyor belt. Fluctuations in mositure content can also be caused by lumping and the like.
In the case of measurements in media having higher temperatures, it has for many years been impossible, by electronic methods, to effect temperature-compensation in the oscillatory circuit of the probe, so that here again unreliable measurements have occurred. However, since it is important with many mixtures to be able to determine the temperature of the mixture for the purpose of ascertaining the required amount of moisture, temperature sensors have also been provided on conveyor belts at positions considerably in advance of, or to the rear of, the moisture-content measuring probe. Temperature sensors are generally of fairly considerable mass in order to reduce wear and tendency to breakdown. Then however not only does inertia occur in the measuring system, but the temperature measurement is also falsified as a result of the fact that the measuring sensor itself requires a long time to reach the high temperature of the material on which the measurements are being carried out, since after each measurement it does not cool down again to the initial temperature.
A particularly disadvantageous feature associated with the known moisture-content measuring methods and apparatuses is the danger of substances adhering to the probe and of its becoming contaminated, particularly after repeated and/or automatic use. This difficulty is repeatedly observed in the case of sticky dirty materials.